(a) Field of the Invention
The present invention relates to a device for measuring the amount of flow of air or other gas being sucked into an internal combustion engine or other unit, said device being arranged so that in order that, for example, the weight ratio of suction air to fuel may be accurately controlled so as to be kept at a constant value, the difference in the suction air pressures existing on opposite sides of an air throttle valve is kept constant by a feedback control device using a fluid, the amount of incoming air being determined by the opening area of said throttle valve.
More particularly, the present invention relates to a device for measuring the weight of air or other gas being sucked into an internal combustion engine or other unit by automatically compensating for the influence of density variations due to the gas temperature and pressure variations.
(b) Description of the Prior Art
In internal combustion engines, e.g., for automobiles, it is particularly important to engine efficiency and exhaust gas coutermeasure to accurately control the weight ratio of suction air to fuel so as to keep it at a constant value. To this end, a high precision measuring device for measuring the amount of suction air is required. Generally, the conventional suction air flow measuring device for this purpose comprises a flow control valve placed in an air suction pipe, a flow detection valve placed upstream of said flow control valve, said two valves defining an intermediate chamber, and means for controlling the pressure in said intermediate chamber so as to keep constant the difference in the pressures existing on opposite sides of said flow detection valve, so that the amount of suction air is proportional to the opening area of the flow detection valve, the amount of flow of air being thus determined by said opening area. The so-called area flowmeter system is known. The control of the flow detection valve by this system is performed by employing a pressure difference control servo-mechanism based on the feedback system wherein when the difference in the pressures existing on opposite sides of the valve is deviated from a predetermined value, the deviation is detected by the pressure difference setting diaphragm of the servo-mechanism, the detected value being then amplified by a fluid mechanism, the resulting amplified output being used to increase or decrease the degree of opening of the flow detection valve so as to bring said pressure difference to said predetermined value.
However, the suction air varies in its specific gravity with its pressure and temperature. Therefore, in order to accurately measure the amount of flow of air, a mechanism which compensates for temperature and pressure is required. Such mechanisms which have heretofore been used are in two types; electrical and mechanical. In either type, there are problems in terms of accuracy and cost. Japanese Patent application No. 107455/73 proposes this kind of compensation mechanism using the above-described feedback control mechanism having incorporated therein a compensation section designed so that compensation for pressure is made by a pressure sensitive device and compensation for temperature is made by a heat sensitive device. The mechanism, however, has problems in terms of the number of parts and construction.